Certain clinical and morphologic findings are described in 55 patients whose hearts at necropsy contained so much fat that they floated in water. The patients were 47 to 89 years old (mean 67). Symptomatic coronary heart disease was present in 28 (51%) and valvular heart disease (mitral stenosis) in 3 (5%). The heart at necropsy was enlarged (greater than 350 g for women and greater than 400 g for men) in 45 patients (82%). The mean heart weight for the 31 women was 470 g and for the 14 men, 515 g. In addition to the severe increase in fat in the atrioventricular sulci and over both ventricles, the amount of fat in the atrial septum was increased in all patients. In 14 patients (25%), the thickness of the atrial septum cephaled to the fossa ovale was greater than or equal to 2 cm. Excessive fat in this location is called "lipomatous hypertrophy of the atrial septum." Of the 16 patients (29%) with fatal acute myocardial infarction, 7 (44%) had rupture of either the left ventricular free wall or ventricular septum. The high frequency of cardiac rupture in these patients supports the contention that rupture during acute myocardial infarction is more common in the fatty than in the non-fatty heart.